1. Field of the Invention
The present invention relates to a converter for the efficient conversion of the nitrogen oxides present in the exhaust gases which issue from an internal combustion engine or the like.
2. Description of the Prior Art
Recently, atmospheric pollution has become a problem of increasing public concern. Thus, there has been an increasing demand for solutions to the problem of exhaust gases which cause atmospheric pollution. Of particular concern is the problem of converting nitrogen monoxide and hydrocarbons which are deemed as the cause of photochemical smog to nontoxic substances. Various approaches have been made for solving this problem. Particularly, various techniques have concentrated on the general approach of purifying the exhaust gases from the internal combustion engine of an automobile or the like.
The composition of the exhaust gases from the internal combustion engine, such as from an automobile using gasoline as fuel, varies with the engine running conditions such as idling, acceleration and deceleration. In general, the composition of such exhaust gases consists, in volume percent, of from 11 to 15% carbon dioxide gas, from 11 to 15% steam, from 0.2 to 4% oxygen, from 0.2 to 8% carbon monoxide, from 0.4 to 4% hydrogen, from 0.06 to 0.006% hydrocarbons such as methane, propane and the like, from 0.1 to 0.3% nitrogen oxides (the majority of which is nitrogen monoxide), the balance being nitrogen. Of these components, carbon monoxide, hydrocarbons and nitrogen oxides are considered to be toxic components, and various attempts have been made to convert the gaseous components into nontoxic materials to minimize the amounts of toxic components discharged into the atmosphere.
In catalytic converters, as in the present invention, the conversion of gaseous components is achieved by converting carbon monoxide and hydrocarbons into carbon dioxide and water by oxidation. Further, these converters convert nitrogen oxides to nitrogen and oxygen by reduction, and subsequently use the oxygen obtained to oxidize the carbon monoxide to carbon dioxide.
While catalysts have been developed which are useful for the oxidation of exhaust gas components, no catalysts have been developed which are effective as reduction catalysts for the conversion of nitrogen oxides.
The catalysts which have been useful in the prior art for the conversion of the toxic components present in exhaust gases include palladium, platinum, platinum plus palladium and platinum plus rhodium on suitable carriers such as alumina. These catalysts serve well as oxidation catalysts, but fail to act as catalysts for reduction of nitrogen oxides. In other words, these catalysts decompose the nitrogen oxides by a reductive reaction, but simultaneously convert portions thereof into ammonia, resulting in the failure to totally purify the exhaust gases. In more detail, the ammonia which has been by-produced will be discharged into the atmosphere, or otherwise will be reoxidized to nitrogen monoxide during its passage through the oxidizing catalyst layer for oxidizing carbon monoxide and hydrocarbons, followed by discharge into the atmosphere. Thus, the intended purification of the exhaust gases is not attained because of the presence of ammonia in the gases.
The amount of nitrogen monoxide produced in the exhaust gases of an automobile depends on the operating conditions of the engine. When the air/fuel ratio is increased at the high operating speeds of an engine, an increase in the amount of nitrogen monoxide results. Also, in such cases, an excessive amount of oxygen is contained in the exhaust gases over that amount which is required to completely burn the components which have not been completely burned in the engine. Under these conditions, the oxygen ratio (which will be described in detail hereinafter) ranges from 100 to 200%. Such high level of oxygen is unfavorable for the conversion of nitrogen monoxide to nitrogen and oxygen. Thus, difficulties have been encountered in developing catalysts useful for the conversion of nitrogen monoxide even under these circumstances.
Thus, in the conversion of nitrogen oxides in the exhaust gases, it is preferred that the aforesaid conversion process be conducted in an efficient manner even at high air/fuel ratios, and also that the amount of by-produced ammonia be minimized.